Cross-disperser gratings

cross-disperser wheel
(click for larger)

One of the major aspects of the CRIRES+ upgrade can be seen in this image. It shows the cross-disperser wheel that holds six reflective gratings, one for each of the near-infrared bands YJHKLM. Visible are the ones for K, L and M bands. The wheel will rotat to choose the band to be observed. The light that reflects off the grating will be dispersed by wavelength before it reaches the main grating with much higher resolution.

Since the direction of cross-dispersion is perpendicular to the main dispersion, we can fit 9 spectral orders onto the detectors (see simulations), compared to a single one before. The simultaneous wavelength coverage and efficiency of CRIRES+ will therefore go up by about an order or magnitude.